Internet protocol concepts and overview
- 1. Internet Protocol Concepts and Overview IP is a very important protocol in internetworking; it wouldn't be an exaggeration to say that you can't really comprehend modern networking without a good understanding of IP. Unfortunately, IP can be somewhat difficult to understand. This is probably because due to its importance, a large amount of complexity has become associated with the protocol over the years, to allow it to meet the many demands placed upon it. Before diving into the details of how IP works, I feel it is worthwhile to take a high-level look at IP and what it does. In this section I provide a brief examination of basic concepts related to the Internet Protocol and how it works. I begin with an overview of IP, how it operates in basic terms and the most important characteristics of how it does its job. I then expand on this discussion by describing the main functions of the Internet Protocol, which can be used as an introduction to the remainder of the sections that explain IP in considerably more detail. I conclude with a brief look at the history of development of IP, its versions, and how it has spawned the development of several IP-related protocols. TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4) The TCP/IP protocol suite is largely defined in terms of the protocols that constitute it; several dozen are covered in this Guide. Most of the critical protocols of the suite function at the lower layers of the OSI Reference Model: layers 2, 3 and 4, which correspond to the network interface, internet and transport layers in the TCP/IP model architecture. Included here are the all- important Internet Protocol (IP) at layer 3 and Transmission Control Protocol (TCP) at layer 4, which combine to give TCP/IP its name. Due to the importance of these and other TCP/IP protocols at the lower layers, this is the largest chapter of The TCP/IP Guide. It contains four subsections. The first describes the two TCP/IP protocols that reside at the network interface layer, layer 2 of the OSI model: PPP and SLIP. The second describes a couple of “special” protocols that reside architecturally between layers 2 and 3: ARP and RARP. The third covers the TCP/IP internet layer (OSI network layer, layer 3), including IP and several other related and support protocol. The fourth describes the TCP/IP transport layer protocols TCP and UDP.
- 2. TCP/IP Internet Layer (OSI Network Layer) Protocols The first two layers of the OSI Reference Model, the physical layer and data link layer, deal primarily with physical network details. The various LAN, WLAN and WAN protocols function primarily at these two layers to connect devices to create networks, and perform functions such as physical connection and signaling, media access control and local delivery of data between devices on the same network. Above these layers, we move beyond the hardware aspects of networking and closer to the more abstract realm of software-related network functions. The third OSI layer is the network layer. We are of course talking about networks in this Guide, and it is no coincidence that the layer bearing that name is one of the most important in comprehending how networks function. It is here that we find protocols that tie networks together to create internetworks, and also where cross-network addressing and routing are performed. The network layer is also called the internet layer in the TCP/IP model. In this section I provide details for the various TCP/IP protocols that reside architecturally at the TCP/IP internet layer / OSI network layer. Much of the focus here is on the all-important Internet Protocol; the section covering IP includes extensive coverage of IP version 4, IP version 6, and IP-related protocols such as IPSec, Mobile IP and IP Network Address Translation (NAT). The following three subsections cover IP support protocols such as the Internet Control Message Protocol (ICMP) and IPv6 Neighbor Discovery (ND) protocol, and the complete set of IP routing protocols. Internet Protocol (IP/IPv4, IPng/IPv6) and IP-Related Protocols (IP NAT, IPSec, Mobile IP) The idea of singling out any one protocol as being more important than the others in a network is kind of pointless, if you think about it. The protocols and technologies work as a team to accomplish the goal of communication across the network. Like any team, no single member can get the job done alone, no matter how good they are. Still, if we were to try to pick a “most valuable player” in the world of networking, a good case could be made that we have it here in this section: the TCP/IP Internet Protocol (IP). Even though it gets “second billing” in the name of the TCP/IP protocol suite, IP is in fact the “workhorse” of TCP/IP. It implements key network-layer
- 3. functions including addressing, datagram handling and routing, and is the foundation upon which other TCP/IP protocols are built. Even the ones lower in the TCP/IP architecture such as ARP and PPP are easier to understand when you know how IP works. In addition to the main functions implemented by the IP protocol itself, there are also several protocols that have been developed over the years that I call “IP-Related” because they are based on IP but add new functionality or capabilities for special purposes. In this section I provide considerable coverage of the TCP/IP Internet Protocol and to several protocols that are closely related to IP. I begin with a section that takes a general look at IP concepts and provides an overview of how IP works. I then have a large section that looks at IP version 4, the current version of the protocol that is in use on TCP/IP networks everywhere. Following this I look at the “next generation” of IP, IP version 6. I then provide sections covering three IP-related protocols: the IP Network Address Translation protocol (IP NAT), the IP Security protocol set (IPSec), and the adaptation of IP for mobile devices (Mobile IP). IP Security (IPSec) Protocols One of the weaknesses of the original Internet Protocol is that it lacks any sort of general purpose mechanism for ensuring the authenticity and privacy of data as it is passed over the internetwork. Since IP datagrams must usually be routed between two devices over unknown networks, any information in them is subject to being intercepted and even possibly changed. With the increased use of the Internet for critical applications, security enhancements were needed for IP. To this end, a set of protocols called IP Security or IPSec was developed. In this section I provide a brief description of IPSec concepts and protocols. I begin with an overview of IPSec, including a discussion of the history of the technology and defining standards. I describe the main components and protocols of the IPSec suite, and its different architectures and methods for implementation. I then move to actually discussing how IPSec works, beginning with a description of the two IPSec modes (transport and tunnel) and how they differ. I describe security associations and related constructs such as the Security Parameter Index (SPI). The last three topics cover the three main IPSec protocols: IPSec Authentication Header (AH), IPSec Encapsulating Security Payload (ESP) and the IPSec Internet Key Exchange (IKE).